Modern electronic systems are typically assembled from multiple printed circuit boards. Such printed circuit boards, which are commonly referred to as “daughter cards”, contain components thereon, such as integrated circuits. Each daughter card also typically includes one or more connectors that allow the components on the daughter card to communicate with components on the other daughter cards in the system.
One way to interconnect the daughter cards in an electronic system is to utilize a midplane. A midplane is a printed circuit board, typically larger than the daughter cards, to which the daughter cards are connected—by way of connectors on both the daughter cards and the midplane—and that provides conductive paths therein. The conductive paths, which are also referred to as “signal traces”, interconnect and provide communication between the daughter cards in the system. A midplane, as the name implies, provides connectors on both sides, allowing daughter cards to be connected on both sides of the midplane. The midplane can route signals between daughter cards connected on the same side of the midplane or can cross-connect a daughter card on one side of the midplane with a daughter card on the other side of the midplane.
In order to connect a connector to the midplane, holes are conventionally drilled through the midplane. The holes, which are also referred to as “vias”, electrically connect to signal traces in the midplane. The inside walls of the vias are typically plated with a conductive material, such as metal, to provide electrical conductivity. The connector is provided with contact ends, such as press-fit contact tails or SMT (surface mount technique) contact tails, for connecting to the vias.
As electronic systems have become smaller, faster and more complex, this has generally required that midplanes provide more vias and signal traces without increasing in size, or in many instances, while actually decreasing in size. This has introduced significant difficulties in designing and fabricating midplanes, as well as significant difficulties in dealing with electrical noise and other electrical characteristics. Electrical noise is usually considered any undesirable electrical energy in an electronic system, including but not limited to, reflections, electromagnetic interference, mode conversions and unwanted coupling, such as cross-talk.
The trend for smaller, faster and more complex electronic systems has also required connectors to carry more and faster data signals in a smaller space without degrading the electrical characteristics of the signal. Connectors can be made to carry more signals in less space by placing signal conductors in a connector closer together. A major difficulty with placing signal conductors closer together is that electrical noise between the signal conductors increases as the distance between signal conductors decreases and as the speed of the signals increases. In addition, as frequency content increases, there is a greater possibility of energy loss. Energy loss may be attributed to impedance discontinuities, mode conversion, leakage from imperfect shielding, or undesired coupling to other conductors (crosstalk). Therefore, connectors are designed to control the mechanisms that enable energy loss. Conductors composing transmission paths are designed to match system impedance, enforce a known propagating mode of energy, minimize eddy currents, and isolate alternate transmission paths from one another. One example of controlling energy loss is the placement of a conductor connected to a ground placed adjacent to a signal contact element to determine an impedance and minimize energy loss in the form of radiation.
One way to control electrical noise in a connector is to utilize differential signals. Differential signals are signals represented by a pair of signal conductors, called a “differential pair”. The voltage difference between the pair of signal conductors represents the signal. If electrical noise is electromagnetically coupled to a differential pair, the effect on each signal conductor of the pair should be similar. This renders a differential pair less sensitive to electrical noise as compared with a single signal conductor. However, use of a differential connector, especially in a midplane system architecture, introduces further difficulties as vias corresponding to the differential pair on either side of the midplane must each be electrically connected in the midplane and signal traces can only be routed between adjacent differential pairs.
What is desired, therefore, is to provide a midplane and a differential connector designed for such a midplane that addresses the difficulties described above.